Amb builder wannabe seeking more experienced heads

For those building and repairing amps on their own. Learna and share ideas.

Re: Amb builder wannabe seeking more experienced heads

Postby pdf64 on Fri May 09, 2014 10:11 pm

You know about the voltage doubler option if the bias supply voltage is marginal?
It's a common thing with SC 50+, which results in the output tubes running too hot, though I've no experience of the 120 models.

Instability / oscillation is a big problem with both high power amps and high gain pre-amps.
If it's free running oscillation in the audio range then it's fairly obvious, but parasitic or super audio oscillation really needs a scope and dummy load to enable it to be identified.
Pete
My band:-http://www.youtube.com/user/RedwingBand
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Sat May 10, 2014 8:33 am

The SC 120 had been extensively serviced before developing its most recent issue so I think it's fine. The problem I fixed was pretty straightforward, the wire from the mains fuse was touching the plate eyelet on one of the EL34 bases and melted through the jacket, causing a short which fried the valve base and killed the insulation between the pins. Easy enough job although I was careful to test the PT secondaries to make sure no further damage had been done. Working great now.
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Thu May 22, 2014 8:34 pm

Hey guys. Well I got the amp up and running after my major surgery. It wasn't all smooth sailing, first I got the bias circuit wrong resulting in a massively excessive voltage on the grids. Then, once I got that fixed I was getting some sound through, but very faint and kinda crappy sounding which turns out to be caused by my wiring the phase inverter incorrectly. So I took that section apart again today and now we're getting sound! I'm not massively enamoured of the tone stack I used for this though, so I'm thinking I'll probably mod that back to the more traditional design. And there's other bits that don't seem to do exactly what I had in mind. But sound is coming through and sounding okay with no unusual hums or buzzes or screeches so it's in a good place to tinker with now.

Here's a pic of the innards as I left them. https://www.dropbox.com/s/udfsn4y5ujuwe ... .42.47.jpg
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Re: Amb builder wannabe seeking more experienced heads

Postby GiF on Thu May 29, 2014 10:03 am

Another update. Me playing some lame guitar through the amp: https://www.youtube.com/watch?v=bIYfUcHm6QA

Obviously it's only single channel but I made a variable input attenuator pedal a while back so I use that to dial back the gain. Since making the video I added a master volume pot so I can control the preamp drive without blowing the roof off.
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Re: Amb builder wannabe seeking more experienced heads

Postby ThisLifeILead on Thu May 29, 2014 2:04 pm

:thumbsup:
"Great spirits often encounter violent opposition from mediocre minds" - A.Einstein
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